Multilayer dielectric filter optical interference filters have been known for many years. Developments in their technology have recently made it possible, by the use of high energy deposition processes such as sputtering or ion-assisted deposition, to produce high quality stable narrow-passband optical interference filters with centre wavelengths that exhibit negligible sensitivity to humidity. Moreover, if the dielectric layers that go to make up the filter are deposited on a substrate having the appropriate temperature expansion coefficient, a temperature coefficient of centre wavelength shift of the filter can also be made by very small, typically less than 2 pm/.degree. C. The construction of a wavelength division demultiplexer using a set of such filters, each with a different centre wavelength, is for instance described in `Fiber Optics Handbook for Engineers and Scientists,` Editor F C Allard published by McGraw-Hill. In chapter 3 of this book P Morra & E Vezzoni describe, with particular reference to its Figure 3.80, a demultiplexer in which a set of dielectric interference filters are mounted, regularly spaced, in two lines on opposed faces of a glass block. An input fibre with a collimating graded-index lens termination directs light through the block to be incident obliquely upon the first filter at the appropriate angle to ensure that light, of a wavelength reflected by all the filters of the set, is reflected so as to be similarly obliquely incident in turn upon each of the other filters of the set. Associated with each filter is an output fibre with a similar collimating graded-index lens termination mounted so that its lens termination receives the transmitted component of the light obliquely incident upon that filter. Each of the collimating graded-index lens terminations is shown as being located in appropriate orientation with respect to the glass block and its filters by means of an associated wedge-shaped spacer.
In principle the structure is quite simple and elegant; in practice there are considerable difficulties in implementing such a structure with satisfactory manufacturing yield and with an acceptable degree of precision and long-term stability to suit dense wavelength division multiplexing (DWDM) applications. In this respect it is to be noted that, since all the filters are mounted in nominally fixed orientation with respect to each other, while there may be limited scope for altering the angle of incident upon the first filter of the set for the purpose of fine-tuning to centre wavelength, no corresponding facility is then possible for further fine-tuning the centre wavelengths of any of the succeeding filters of the set. Then there is the additional problem that individual filters are subject to alignment errors as the result of the trapping of dust particles and the like between the filters and the glass block. A simulation of the effects of such particles upon filters considered to be perfectly prepared at the correct centre wavelength spacing and then bonded on to the block indicated that particles of only 2 .mu.m diameter are liable to introduce errors in channel position exceeding a 100pm tolerance threshold. These alignment problems can be circumvented by separately bonding free-standing filters by their side edges to a common substrate. Each such filter then has three translational and three rotational degrees of freedom. Then the positioning of the end of each of the collimating graded-index lens terminated fibre on the substrate relative to its associated filter involves a further three translational and three rotational degrees of freedom. A certain number of these degrees of freedom are of little or no practical consequence, examples being the translational movement of a lens terminated fibre along its axis, and rotation of the fibre or its associated filter about that axis. There remain however a large number of degrees of freedom for which such scale movement can have a significant effect upon the optical operation of the device.